A plastic optical fiber comprising a core and a cladding both of which comprise a plastic is rather suitable for an optical transmission line to be used for a short distance such that the transmission loss in the line disposed between devices for sending and receiving an optical signal poses substantially no problem (for example, that between electronic devices), as compared with a glass optical fiber. In addition, since a plastic optical fiber can generally be produced at a lower cost than that of a glass optical fiber, such a fiber is widely used as an optical transmission line for a short distance. Thus, the importance of a plastic optical fiber has been increased, particularly in view of a project or design for a next-generation communication network such as LAN (local area network) and ISDN (integrated service digital network).
Heretofore, there has been put into practical use a plastic optical fiber 1, as shown in a schematic perspective view of FIG. 23, which comprises a core 2 comprising a resin such as PMMA (polymethyl methacrylate resin), PC (polycarbonate resin), or a copolymer thereof; and a cladding 3 comprising a fluorine-containing resin, etc., and has a refractive index distribution (profile) as shown in FIG. 24, i.e., a step-index (SI) type optical fiber.
In addition, as an optical fiber which is capable of transmitting a larger quantity of information per unit time than the above-mentioned SI-type optical fiber, there has been proposed a graded-index (GI) type optical fiber having a refractive index distribution as shown in FIG. 25. Such a GI-type optical fiber is disclosed in Japanese Patent Publication (KOKOKU) Nos. 5857/1977 (Sho 52-5857) and 30301/1979 (Sho 54-30301), and Japanese Laid-open Patent Application (KOKAI) Nos. 130904/1986 (Sho 61-130904) and 162008/1986 (Sho 61-162008). However, these optical fibers still have various unsolved problems in view of the production thereof, etc., and an optical fiber having a desired property has not been obtained yet.
More specifically, in the conventional plastic optical fibers, since the refractive index distribution has been formed by utilizing a special chemical reaction such as one employing a difference in reactivity, and one employing a gel effect, there has been a severe limitation on the conditions such as the size of a preform, and the kind of a material, in view of the provision of a desired refractive index. Accordingly, the conventional plastic optical fibers have posed a problem such that mass production thereof is difficult or a fiber material having excellent transmission property and high reliability is difficult to be obtained.
Further, in the conventional optical fibers as described above, it is difficult to freely control the reaction for providing the refractive index distribution, and therefore it has been difficult to obtain a fiber having an ideal GI-type refractive index distribution in a good yield.
Furthermore, Japanese Laid-Open Patent Application No. 16504/1990 (Hei 2-16504) discloses a process for producing a plastic optical fiber by concentrically extruding a laminate product of two or more species of polymerizable mixtures for providing mutually different indices. However, according to the present inventors' investigation, such a process has following problems.
More specifically, since the above process is a laminating-extrusion process and it can provide extrusion steps corresponding to only about ten layers, the resultant product is inevitably caused to have a stepped-type refractive index distribution. When an optical fiber having such a stepped refractive index distribution is used, it is difficult to transmit a large quantity of information. Further, in the above process, it is suggested that a continuous and smooth refractive index distribution can be provided by further diffusing a monomer into the product after the laminating-extrusion process. However, when such monomer diffusion is employed, the number of the production steps is increased and the resultant productivity becomes low. In addition, since the above process includes an operation for conducting monomer diffusion the control of which is difficult, it is difficult to provide an ideal GI-type refractive index distribution.
Further, there is proposed a process for producing a preform for plastic optical fiber having a continuously changing refractive index by repetitively pouring two species of materials having mutually different refractive index differences into a hollow cylindrical member and subjecting such materials to polymerization and lamination under the action of a centrifugal force (as described in Japanese Laid-Open Patent Application No. 119509/1985 (Sho 60-119509)). However, it is not necessarily easy in this process to regulate the resultant refractive index so as to provide a desired and designed value, and the production cost tends to become somewhat higher.
In addition, Japanese Laid-Open Patent Application No. 124602/1993 (Hei 4-124602) discloses a process for producing a plastic optical fiber by subjecting a core material to fiber spinning so as to provide a predetermined diameter, and coating the resultant product with a cladding material. However, in order to produce a GI-type plastic optical fiber by such a coating operation using a cladding material, it is necessary to conduct multiple-stage coating operations, so that the production process becomes complicated.
On the other hand, it has also been proposed that a GI-type preform (base material) is prepared in advance and is subjected to hot stretching to provide a fiber (Polymer Preprints, Japan, vol. 41, No. 7, pp. 2942-2944, Autumn of 1992). It is conceivable that such a process can reduce the number of production steps and can prepare various species of fibers having different outer diameters. However, according to the present inventors' investigation, in such a process wherein the GI-type preform is simply inserted into a drawing furnace to be subjected to fiber drawing, a fluctuation in the outer diameter after the fiber drawing is liable to occur and the strength of the resultant fiber thus obtained tends to become lower than that of fiber produced by other process.
In general, in a plastic optical fiber to be actually used, a cover portion called as a jacket layer or a sheath layer is further provided on the above-described cladding layer so as to protect the main body of the plastic optical fiber (as disclosed in FIG. 1 of Japanese Laid-Open Patent Application No. 230104/1985 (Sho 60-230104)). As disclosed in Japanese Laid-Open Patent Application Nos. 178302/1983 (Sho 58-178302) and 57811/1985 (Sho 60-57811), etc., a fiber having a good heat resistance and a good weathering resistance can be provided by selecting a jacket or sheath material having a good heat resistance and a good weathering resistance.
Further, Japanese Laid-Open Patent Application No. 190204/1992 (Hei 4-190204) discloses a technique such that an inorganic filler is incorporated into a jacket layer so as to facilitate the formation of metal plating on the surface of the jacket layer.
In these conventional methods, an optical fiber preform is drawn into a fiber shape and then a cover portion is formed on the resultant fiber by utilizing a method such as die coating and extrusion, and therefore the formation of the cover portion is troublesome and the productivity becomes low, whereby the resultant production cost becomes high.
It is also theoretically conceivable that, instead of the provision of the above-mentioned cover portion, the main body of the optical fiber is caused to have a considerably large outer diameter so that the cladding layer may also function as a protecting layer. In this case, however, there is posed a new problem such that the amount of a plastic material to be used per unit length is increased. Further, since the plastic material to be used for a plastic optical fiber is one having a high optical transparency which has been produced so as to provide a high purity through purification, and has a high production cost, an increase in the amount of such as plastic material to be used per unit length constitutes a factor of an increase in the production cost.
An object of the present invention is to provide a plastic optical fiber or a preform therefor, and a process and an apparatus for producing such a plastic optical fiber or a preform, which have solved the above-mentioned problems encountered in the Background Art.
Another object of the present invention is to provide a process and an apparatus for producing a plastic optical fiber or a preform therefor, which has a desired refractive index distribution and is capable of being produced easily.
A further object of the present invention is to provide a process and an apparatus for producing a plastic optical fiber or a preform therefor, which has a desired refractive index distribution and is capable of being produced at a low cost.
A further object of the present invention is to provide a (drawing) process for producing a plastic optical fiber which can maintain a sufficient mechanical strength and can assure a long-term reliability after the fiber formation.
A further object of the present invention is to provide a (drawing) process for producing a plastic optical fiber which can suppress a change in the outer diameter thereof due to heat and can assure a long-term reliability after the fiber formation.
A further object of the present invention is to provide a process for producing a plastic optical fiber having a desired refractive index distribution and having a jacket layer which is capable of being easily formed.
A further object of the present invention is to provide a process for producing a plastic optical fiber having a desired refractive index distribution and having a jacket layer which has been reduced in the production cost.